50W Plexi-Style Circuit Build Completed

I started looking into building a bass-spec Plexi circuit started when I first came across the Metro Amp 50W build guide. I read about amps that switched between different Marshall eras, like the Heritage Colonial, Aiken Intruder, Germino Club 40, and Fargen Mini-Plex.

The form factor was definitely inspired by a one-off JTM50 build on the Lil Dawg Amps blog. It showed me that the space spacing layout of a Marshall circuit in a 17” wide head was possible. My lunchbox 5F1/5F2A head was also influenced by his offerings.

The kitchen sink approach to the switching options was mainly based on a Johan Segeborn video demonstrating a Marshall 1987X that  covered several Marshall circuits. I can get close to Bass Spec, Organ Spec, and Lead Spec ('67, '68, '69, '70).

I ordered the following tubes for the amp from Doug's Tubes:

• V1 - Ruby 12AX7AC5 HG+
• V2 - Shuguang 12AX7
• V3 - Sovtek 12AX7LPS
• V4, V5 - Electro Harmonix EL34

I will explore more fancy tubes in the future, but purchased these to make sure the amp would fire up.

Front panel of the amp.

Front Panel Option Summary:

• Switchable 470pF mixer capacitors on each channel
• Either Channel 1 or 2 can be set as the bright channel
• Channel 1 Bright Cap Select: 500pF, none, 100pF
• Channel 2 Bright Cap Select: 4700pF, none, 250pF
• With the Channel 2 mixer cap engaged, I have a shared cathode bright channel with more bright cap options
• PI Coupling Cap Value Select: 0.022uF, 0.09uF
• Switchable between Lead and almost Bass spec

Rear panel of the amp.

Rear Panel Option Summary:

• V1 split/shared cathode switch
• V1A Cathode Bypass Cap Value Select: 221uF, 1uF, 5.7uF
• V2A Cathode Bypass Select: 330uF, none, 0.68uF
• Lead/Bass Tonestack Switch
• Variable Negative Feedback: 47k to 147k on 8 ohm tap
• Bias Test Points

Transformers:

• Power - Classictone 40-18095
• Output - Classictone 40-18025

Modifications shown on Metro Amp's 50W Plexi Schematic.

Bass Spec: changing through different bright caps.

Lead Spec: Trying out the different switch options.

Lead Spec: All controls at maximum.

Older Posts:

https://outmodedelectronics.blogspot.com/2019/02/start-of-60s-plexi-50w-inspired-amp.html

https://outmodedelectronics.blogspot.com/2019/04/continuation-of-plexi-inspired-build.html

https://outmodedelectronics.blogspot.com/2019/06/1987-and-5e3-turret-boards.html

https://outmodedelectronics.blogspot.com/2019/07/incremental-progression-on-amp-builds.html

https://outmodedelectronics.blogspot.com/2019/08/plexi-amp-build-chassis-mounting.html

https://outmodedelectronics.blogspot.com/2019/10/finishing-up-plexi-build.html

Finishing Up Plexi Build

I finished soldering all of the components and will begin checking all connections prior to the initial power-up.

I have ordered 3 12AX7s and 2 EL34s for testing of the amp next week.

Chassis Wiring

Front panel without knobs.

Rear panel without knob for NFB.

Top view of chassis.

Edit 10/31/19. I successfully powered up the amp with all tubes installed.

V1: Ruby 12AX7AC5 HG+
V2: Shuguang 12AX7
V3: Sovtek 12AX7LPS
V4, V5: Electro Harmonix EL34 matched pair, average distortion

B+ voltage off the HT fuse is nominally 432V, in line with typical '68 Plexi 50W voltages.

Below are some additional photos of the completed amp.

Front view with cage and knobs installed. I will redo the Midde and Treble Dymo labels.

Output section wiring.

Power wiring.

Input section wiring.

Switchable mod wiring.

Tone control wiring.

Initial observations playing through IRs loaded by Suhr Reactive Load:

• The bright caps increase the noise floor of the amp, especially when the channel has a bright mixer capacitor engaged. Adding gain through the V2A cathode bypass cap does not result in as much noise.
• Engaging a bright cap with the bright mix capacitor off on the active channel does not increase the noise floor as much.
• I have 2 near-identical channels in shared cathode mode, by engaging the bright mixer cap for the active channel, and turning it off the the unused channel. I can set up four different bright cap values this way.
• The 4700pF bright cap (channel 2) immediately distorts the sound as expected.
• 500pF on a clean sound is too bright, even on shared cathode mode.
• The lead channel in split cathode mode is noisy when I turn it up to 10. This is more gain than I need, and I am ok with the noise floor when I dial the amp back to 8 (Fender-scale).

Plexi Amp Build chassis mounting progress

I found some time to start mounting components onto the Hammond chassis. Most of the front panel is populated, as well as a bit of the backpanel.

One of the on-off-on DPDT switches was a dud, but I had luckily ordered a spare.

Progressing panel component and transformer mounting.

A shot of the front panel from the underside.

Shortening some of the transformer leads.

Testing the fit of the main turret board.

Update 9/4/19: more wire dressing and a few solder joints.

Update 9/10/19: I had half an hour to work: layout heater wiring from V5 to V3.  It’s more time consuming than I had imagined. 

Update 9/13/19: Heaters installed. I tried to match the installation depicted in the MetroAmp builder guide.

Update 9/26/19: Input jack wiring, more grounds, filter cap wiring, bright cap switches.

Update 10/16/19: soldered many of the wires on the main turret board side. 

Update 10/18/19: Connected turret board to front panel controls and input jacks, laid out mod components on the turret strips.

AutoCAD layout used for component placement.

Incremental Progression on Amp Builds

I finally got all the components for the Marshall-inspired build. I had to reorder and redrill the chassis due to a mistake in the layout. 

Populated 1987 Turret Board

Populated 5E3 Turret Board

Hammond Chassis Laid Out for Drilling

Top View of Drilled Chassis

Fitting the cage to drill cage mounting holes

Power Transformer Mounting Holes

Output Transformer Mounting Holes

Plexi and 5E3 Turret Board Progress

As I upgraded some of the 2W metal film resistors to 3W on the 1987 turret board, I started populating a board for a future 5E3 head build that I have been putting off.

All passive components are now ordered except for the filter capacitors and a diode for the 1987 board. I am now considering installing a hi/low B+ voltage switch on the Plexi. I plan on using the 625V tap on the ClassicTone 40-18095 power transformer, but am thinking about utilizing the 690V or 560V taps.

The 5E3 will likely  be a straightforward build, with little or no modifications. Parts will be Vishay metal film resistors, Vishay MKT1813 and Cornell Dubilier 150 film capacitors, F&T filter caps, Cornell Dubilier silver Mica capacitors, and ClassicTone transformers.

Start of Plexi Turret Board Work

I started populating the turret board with the components on hand. There were a couple of parts that had leads that were too short for this board.

• The V1A cathode bypass capacitor is 1uF, with off-board values of 3.3µF and 220µF to be switched in parallel.
• I couldn’t get the 2.7kΩ resistor onto the V1B cathode and will order a 2W metal film resistor for this position.
• The V1B coupling capacitor on the board is 0.0022µF, with 0.022µF switchable in parallel.
• The mix capacitors will be switchable off-board on either channel.

• The V2A cathode bypass capacitor will be switched into the circuit off-board between 0.68µF and 330µF.
• I will switch out the 2W metal film resistors above for the 3W for a more uniform look.
• The tone stack will be switchable between lead and bass. Bass spec values (250pF, 56kΩ) are laid out on the board.
• The 0.022µF coupling capacitors will be switchable to 0.09µF via parallel 0.068µF capacitors.
• The bias pot will be installed on the chassis.
• A diode still needs to be installed. The leads on the 1N4007 that I ordered had were too short for the terminals on the board.

Continuation of Plexi-Inspired Build

The rear of the chassis was drilled out for the power (Neutrik Powercon) main and HT fuses, bias test points, and various tone mods.

The main perfed turret board is from ValveStorm. Black turret strips are from Antique Electronics Supply (Tubes and More).

I am drafting the layout in AutoCAD to tentatively coordinate the various theoretical locations of drilling points. All holes smaller than 3/4” will be hand drilled. Holes for tube sockets and capacitor cans will be made with the corresponding Greenlee hole punches.

Finalized wiring layout to come prior to further drilling of the chassis. I am trying to find time to coordinate turret board mounting with the mods (black turret strips), Larry grounding, and transformer wiring based on the specific Classictone transformers I plan on purchasing.

Start of 60's Plexi 50W-inspired Amp Build

I have started to build a 50W guitar amp, based on typical 1968 Marshall 1987 Lead Spec documentation found on the Internet.

The amp will have front and back panel toggle switches, to have options from the shared cathode Plexis and later Lead Spec Marshall circuits.

I will be using a Hammond 1444-32 chassis and 1451-22BK3 cage. By skipping the traditional Plexi cosmetics (headshell, faceplates, pre-drilled chassis), I hope to keep the total cost of the project at around $700. The initial intent was to take up less space than my Jet City JCA22H.

Further deviations include:

• Omission of standby switch
• Omission of impedance selector (Impedance will be 8 ohms)
• Only 1 speaker output
• Neutrik PowerCon Power Connector
• Bias Test Points and Bias Pot
• Variable negative feedback pot
• Fender amp knobs that go from 1-10

I started the initial drilling of the chassis on 2/6/19.